I usually do this one and it can be a bit of a shambles.
in their FAQs they give the following answers to questions -
Is fancy dress allowed?
Yes but please make sure it is lightweight, practical and runner friendly. The run is in summer, so make sure you're not going to be too hot. You must also wear your race t-shirt as this will be your race number so this number will need to be visible at all times (!!! that would be fun over a costume then!!)
Do I have to wear my raace t-shirt?
Yes. This t-shirt is also your race number so must be worn throughout the race and the number itself must be visible at all times. This is important so we know you are part of the race and services like photos and video will only work if your race number is visible.
I had the same issue with Run to the Beat last year though they did back down and say I could wear something else and just display my number. It's a crazy plan as your shirt might not fit or could be uncomfortable. To send it back and get a different one would cause chaos with the race numbers and leave very little time for you to train wearing the new gear.
Will it be a technical shirt or their usual cotton type thing? No way I would run in their usual offering.
They're not known for being in control too well once you get past the baggage drop so I would say do what you like once you're through. Displaying your number is the important thing.
Also, if they don't tell you about that bit up front they can hardly insist you stick to it.
I'm quite glad I have a clash of events this year !
Good luck